When several optical components are connected in tandem, an optical isolator is usually required to suppress instability of the optical source. Such instability may be caused by light reflected back into the optical source by a neighboring component, or by light originating from a different source that impinges on the optical source.
Prior art optical isolators typically rely on the non-reciprocal nature of magnetic media to achieve isolation. These current devices rely on a polarization rotation or non-reciprocal beam deflection, and with time invariant systems this non-reciprocal behavior is required by definition. Other approaches, such as using a linear polarizer in conjunction with polarization components that function as quarter-wave plates, require that the reflection being isolated against is a non-polarization-changing reflection. Therefore, if the reflected signal is in an arbitrary polarization state, the reciprocal isolation means discussed above are inadequate. Further, interfering light from a different source may have an arbitrary polarization state. Thus, any time-invariant reciprocal means cannot in general provide isolation.
It is known that directly modulated semiconductor laser sources suffer from frequency chirp problems due to the inherent fluctuations of the complex index of refraction that are used to induce the modulation. Semiconductor electroabsorption modulators using bulk or quantum well structures have been developed to mitigate such frequency chirp problems, and monolithically integrated devices comprising electroabsorption modulators and distributed feedback lasers (DFB lasers) or distributed Bragg reflector lasers (DBR lasers) have been demonstrated See, for example, Y. Noda et al., "High-Speed Electroabsorption Modulator with Strip-Loaded GaInAsP Planar Waveguide", IEEE J. Lightwave Tech., Vol. LT-4, pages 1445-53 (1986), and M. Suzuki et al., "Monolithic Integration of InGaAs/InP Distributed Feedback Laser and Electroabsorption Modulator by Vapor Phase Epitaxy", IEEE J. Lightwave Tech., Vol. LT-5, pages 1279-85 (1987). For very high bit rate systems of 10 Gigabits per second or higher, however, even the small amount of frequency chirp exhibited by a monolithically integrated laser and electroabsorption modulator device can degrade system performance. A significant component of the chirp in such a device can result from undesirable reflections from the modulator output facet. In addition, reflections from adjoining components may cause system degradation when an external modulator is used to encode the unmodulated output of a laser source for high-speed, long distance optical fiber data transmission.
Although the monolithic integration of optical devices on the same semiconductor substrate has many advantages including reduced connection losses, increased reliability, lower cost and lower power consumption, due to the reciprocal properties of semiconductor materials and the requirements for isolation outlined above, it has not previously been possible to effectively integrate optical isolators and light sources. Consequently, both reflections from adjoining components and light originating from other sources may degrade the overall performance of the light source.